1. Field of the Invention
The present invention relates to a method for transmitting data in a communication system, and more specifically, a method for transmitting information regarding codewords in a multiple input multiple output system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a popular wireless communication technology used to multiplex data in the frequency domain.
A multiple antenna communication system, which is often referred to as multiple input multiple output (MIMO) system, is widely used in combination with OFDM technology, in a wireless communication system to improve system performance.
In a MIMO system, both transmitter and receiver are equipped with multiple antennas. Therefore, the transmitter is capable of transmitting independent data streams simultaneously in the same frequency band. Unlike traditional means of increasing throughput (throughput is the amount of data transmitted per time unit) by increasing bandwidth or increasing overall transmission power, MIMO technology increases the spectral efficiency of a wireless communication system by exploiting the additional dimension of freedom in the space domain due to multiple antennas. Therefore MIMO technology can significantly increase the throughput and range of the system.
Data transmission in a MIMO system may use a single codeword MIMO transmission scheme or a multiple codewords MIMO transmission scheme.
In the single-codeword MIMO transmission scheme, a single codeword is used for the data to be transmitted. That is, in a single-codeword MIMO transmission scheme, a cyclic redundancy check (CRC) is added to a single data block and then coding and modulation are sequentially performed. Subsequently, the coded and modulated data block is demultiplexed into multiple symbols for transmission over multiple antennas.
In the multiple-codeword MIMO transmission, a single data block is first demultiplexed into smaller stream blocks, and each stream block uses a different codeword. That is, after demultiplexation, individual CRCs are attached to these smaller stream blocks and then separate coding and modulation are performed on these smaller blocks. These smaller blocks are then transmitted via separate MIMO antennas.
For a single-user 4×4 MIMO (i.e., 4 transmitting antennas and 4 receiving antennas) system, the minimum number of MIMO codewords (CW) that can be used is one, while the maximum number of codewords that can be used is two. For rank greater than 1 (i.e., more than one MIMO layer), that is rank-2, rank-3 or rank-4, two codewords are used for data transmission. In case of rank-1 transmission (i.e., one data stream is transmitted), a single codeword (CW1) is used for data transmission. In case of rank-2, CW1 is used for the first layer layer-1 (i.e., first data stream) and CW2 is used for the second layer layer-2 (i.e., second data stream). In case of rank-3, CW1 is used for layer-1 while CW2 is used for both layer-2 and layer-3; or, alternatively, CW1 is used for both layer-1 and layer-2 while CW2 is used for layer-3. In case of rank-4, CW1 is used for layer-1 while CW2 is used for the remaining layers (i.e., layer-2, layer-3 and layer-4); or, alternatively, CW1 is used for layer-1 and layer-2 while CW2 is used for layer-3 and layer-4.
When data is transmitted in a contemporary single-user 4×4 MIMO system, control signals that indicate whether one codeword MIMO scheme or two codeword MIMO schemes is used for the transmission, are coded using a single signal format. This results in inefficient use of the system resources.